1. Field of the Invention
The present invention relates to a process for producing 3-methyl-2-pentyl-cyclopent-2-en 1-one.
2. Description of the Background
Acid cyclodehydration of the readily available alkyl-substituted .gamma.-lactones to form alkylated cyclopent-2-enones has long been known. For example, according to French Pat. 765,515, .gamma.-undecalactone is cyclodehydrated by means of concentrated sulfuric acid to form 2-hexylcyclopent-2-enone. As another example, according to DRP Pat. 639,455, .gamma.-methyl-.gamma.-decalactone (5-hexyl-5-methyl-dihydro-2(3H)-furanone) is cyclodehydrated at 300.degree. C. with silica gel to form 3-methyl (dihydrojasmone). Dihydrojasmone is a known fragrance.
The first process affording high yields was published by C. Rai and S. Dev (J. Indian Chem. Soc. 34, 178 (1957). This process uses polyphosphoric acid in the form of a solid mixture of phosphorus pentoxide and 85% phosphoric acid, but more than 24 mole equivalents are required for complete conversion of the lactone at 97.degree. C. The product is then extracted from the solid polyphosphoric acid. In industrial applications, however, such a process is unusable because of the corrosiveness of hot phosphoric acid melts and also due to problems caused by disposing phosphoric acid waste water contaminated with organic compounds. Thus, this conventional process is plagued by the use of solid phosphoric acid as a cyclodehydration reagent wherein large reagent quantities and long reaction times are required. This process is also attended by corrosion and waste water contamination with organic compounds.
A liquid reagent developed by P. E. Eaton et al. (J. Org. Chem. 38, 4071 (1973) consisting of 10 parts by weight of methanesulfonic acid and 1 part by weight of phosphorous pentoxide also entails considerable technical problems. First, in order to convert .gamma.-methyl-.gamma.-decalactone into dihydrojasmone, about 80 times the amount by weight of this reagent is needed. Second, the mixture is so aggressive that it can only be used at room temperature necessitating the use of long reaction times, such as 33 hours. Third, the extraction of the water-soluble methanesulfonic acid with organic compounds creates contaminated waste water.
German Pat. Discl. 24 39 742 describes, among other things, the cyclodehydration of .gamma.-lactones in solid acids at high temperatures. For example, .gamma.-methyl-.gamma.-decalactone is converted by flash pyrolysis with borophosphate heated to 350.degree. C. to form a mixture containing 84 GC-% dihydrojasmone and 4 GC-% adduct. Although this process uses only 10% by weight of catalyst compared to the lactone, the exchange and disposal of the solid, polyester-attached catalyst is, nonetheless, very cost-intensive in practice.
Thus, a need continues to exist for a process for producing 3-methyl-2-pentyl-cyclopent-2-en-1-one by the cyclodehydration of .gamma.-methyl-.gamma.-decalactone that avoids the above disadvantages.